Melt flow control valves of the above mentioned general type are typically used in extrusion plants especially for controlling the flow of the polymer melt to large-surface-area melt filters.
Such valves are conventionally embodied as three-path valves and serve to control the flow of the melt into and out of the respective filter chamber or chambers. In double melt filter arrangements, one filter chamber is respectively operational at any given time, i.e. being used for filtering the melt during the extrusion production process, while the other filter chamber is being cleaned and equipped with a new filter cartridge. The used filter cartridge is cleaned in a space or chamber that is especially provided therefore and is then prepared for reuse.
Control of the melt flow respectively to the two filter chambers is typically provided by two conventional three-path valves, which are available and are utilized in different embodiments having single cylindrical or conical cock plug bodies, double cylindrical or conical cock plug bodies, spool-type slider valves with a plate construction or spool-type slider valves with cylindrical valve stems or valve bodies.
Spool valves or slider valves provide a disadvantageous flow guidance of the melt and furthermore have relatively long melt flow paths in comparison to the cock-type valve embodiments. Such long flow paths cause problems in cleaning, waste of material, and difficulty in controlling the characteristics of the melt material during its rather long passage through the valve.
While the conventional cock-type valves at least partially avoid the above mentioned problems of slide-type valves, they suffer other disadvantages as follows. Previously known cock-type valves with a rotational drive in an axial direction of the cock plug body conventionally have a radially directed melt inlet as well as a radially directed melt outlet. When such valves are installed in connection with typical vertically-oriented melt filter arrangements, it is disadvantageously necessary to provide respective separate valve drives from above the filter and from below the filter. With such an arrangement, it is not possible to achieve a synchronization of both valves by using a rigidly interconnected shaft, for example, especially due to the relatively high rotational moments and the opposite rotation directions that are involved in the two valves. Moreover, the space is rather limited, especially underneath the filter arrangement, and for this reason, problems arise in the installation, operation and maintenance of the respective plant utilizing such conventional embodiments of melt flow control valves having axial rotational drives.
In double cock-type valves, both the melt inlet as well as the melt outlet are provided and controlled by a single cock, whereby operating advantages are achieved, but the melt feed channels and melt lines need to be substantially longer and therefore the additional cleaning effort and complication is considerably increased.